1. Field of the Invention
The subject invention is directed to a diffractive phased array assembly used as a beam steering device in an optical system, and to a method of fabricating and constructing such an assembly.
2. Description of Related Art
Diffractive phased array assemblies are used as beam steering devices in many optical systems such as bar code scanners, laser machining applications and laser printers, as discussed in U.S. Pat. No. 6,480,334 to Farn, the disclosure of which is herein incorporated by reference in its entirety.
A known diffractive phased array design used as a beam steering device in LIDAR (Laser Imaging Detection and Ranging) applications is formed from 2-inch square silicon plates. The surfaces of these plates are patterned with complimentary microscopic grating features that are used to steer a monochromatic laser beam of a prescribed wavelength when the array plates are placed in intimate contact and one plate is transversely displaced with respect to the other.
In the current state of the art design, which is shown for example in FIG. 1, the plate thickness d is about 1-2 mm, the plate spacing g is approximately 1 μm and the depth of the grating features is approximately 0.5 μm. This design yields a theoretical 0th-order optical throughput that is close to unity and remains above 50% for angles θ as large as +/−20°. However, the throughput falls off dramatically for gap sizes that are greater than 1 μm.
The sensitivity of the optical throughput to the localized plate spacing significantly limits the usefulness of this state of the art design. Moreover, substrate imperfections such as warp and wedge non-uniformities can be significantly larger than 1 μm across the face of the plate, even for the highest quality material available. Additional warping can occur when the plates are mounted to a translation mechanism used to vary the output beam angle.
Applicant recognizes that the manufacturability of the state of the art assembly can be markedly improved by replacing one or both plates with membrane structures, as shown for example in FIG. 2, which illustrates one device in accordance with the invention. In such an instance, the substrate 210 is removed from the grating area 220, leaving only a membrane 225 of thickness d′ which could be on the order of the plate spacing, as explained in more detail below.